Liquidambar styraciflua, commonly called American sweetgum, sweetgum, sweet gum, sweet-gum[1] (sweet gum in the UK),[2]hazel pine,[3]American-storax,[1]bilsted,[4]red-gum,[1]satin-walnut,[1]star-leaved gum,[4] or alligator-wood[1] is a deciduoustree in the genus Liquidambar native to warm temperate areas of eastern North America and tropical montane regions of Mexico and Central America. Sweet gum is one of the main valuable forest trees in the southeastern United States, and is a popular ornamental tree in temperate climates. It is recognizable by the combination of its five-pointed star-shaped leaves and its hard, spiked fruits. It is currently classified in the plant familyAltingiaceae, but was formerly considered a member of the Hamamelidaceae.[5]

This plant's genus name Liquidambar was first given by Linnaeus in 1753 from [the Latin] liquidus, fluid, and the Arabicambar, amber, in allusion to the fragrant terebinthine juice or gum which exudes from the tree. Its specific epithetstyraciflua is an old generic name meaning flowing with styrax (a plant resin).[6] The names "storax" and "styrax" have long been confusingly applied to the aromatic gum or resin of this species, that of L. orientalis of Turkey, and to the resin better known as benzoin resin from various tropical trees in the genus Styrax.

The common name "sweet gum" refers to the species' "sweetish gum",[7] contrasting with the black gum (Nyssa sylvatica), only distantly related, with which the sweet gum overlaps broadly in range. The species is also known as the "red gum", for its reddish bark.[7]

The earliest known published record of Liquidambar styraciflua is in a work by SpanishnaturalistFrancisco Hernández published posthumously in 1651, in which he describes the species as a large tree producing a fragrant gum resembling liquid amber, whence the genus name Liquidambar. In Ray's Historia Plantarum (1686) it is called Styrax liquida.

An ancestor of Liquidambar styraciflua is known from Tertiary-aged fossils in Alaska, Greenland, and the mid-continental plateau of North America, much further north than Liquidambar now grows. A similar plant is also found in Miocene deposits of the Tertiary of Europe.[8]

The US government distribution maps for this species are incorrect concerning the southern limit of distribution in Florida. This species occurs abundantly at Highlands Hammock State Park, Sebring, Highlands County, FL, and even southwest of Lake Okeechobee. (see the Univ. South Florida Atlas of Florida Plants)[9]

Liquidambar styraciflua is a medium-sized to large tree, growing anywhere from 33–50 feet (10–15 m) in cultivation and up to 150 feet (46 m) in the wild,[10] with a trunk up 2–3 feet (0.61–0.91 m) in diameter, on average.[11] Trees may live to 400 years.[12] The tree is a symmetrical shape and crowns into an egg shape when the branches get too heavy after its first two years of cultivation.[6]

Another distinctive feature of the tree is the peculiar appearance of its small branches and twigs. The bark attaches itself to these in plates edgewise instead of laterally, and a piece of the leafless branch with the aid of a little imagination readily takes on a reptilian form; indeed, the tree is sometimes called Alligator-wood.[8] The bark is a light brown tinged with red and sometimes gray with dark streaks and weighs 37 lbs. per cubic foot.[11] It is deeply fissured with scaly ridges.[8] The branches carry layers of cork.[13] The branchlets are pithy, many-angled, winged, and at first covered with rusty hairs, finally becoming red brown, gray or dark brown.[8] As an ornamental tree, the species has a drawback—the branches may have ridges or "wings" that cause more surface area, increasing weight of snow and ice accumulation on the tree. However, the wood is heavy and hard with an interlocking grain,[6] but is difficult to season.[14]

The leaves usually have five (but sometimes three or seven) sharply pointed palmate lobes.[10] They are 3-5 inches wide on average and have three distinct bundle scars.[15]

Summer foliage of Liquidambar styraciflua

They are long and broad, with a 6–10 cm petiole. The rich dark green, smooth, shiny, star-shaped leaves generally turn brilliant orange, red, and purple colors in the autumn.[6] This autumnal coloring has been characterized as not simply a flame, but a conflagration. Its reds and yellows compare to that of the maples (Acer), and in addition it has the dark purples and smoky browns of the ash (Fraxinus).[8] However, in the northern part of its range, and where planted in yet colder areas, the leaves are often killed by frost while still green. On the other hand, in the extreme southern or tropical parts of its range, some trees are evergreen or semi-evergreen, with negligible fall color. The leaves are three to seven inches broad with glandular serrate teeth. The base is truncate or slightly heart-shaped. They come out of the bud plicate, downy, pale green, when full grown are bright green, smooth, shining above, paler beneath.[8] They contain tannin and when bruised give a resinous fragrance.[14]

Autumn foliage of Liquidambar styraciflua

While the starry five-pointed leaves of Liquidambar resemble those of some maples (Acer), Liquidambar is easily distinguished from Acer by its glossy, leathery leaves that are positioned singly (alternate), not in pairs (opposite) on the stems. Luna and Promethea moth caterpillars feed on the leaves.[16]

The flowers typically appear in March to May and persist into Autumn, sometimes persisting into the Winter. They are typically about 1–1.5 inches in diameter and are covered with rusty hairs.[11] The flowers are unisexual[16] and greenish in color. Staminate flowers in terminal racemes two to three inches long, the pistillate in a solitary head on a slender peduncle borne in the axil of an upper leaf. Staminate flowers destitute of calyx and corolla, but are surrounded by hairy bracts. Stamens indefinite; filaments short; anthers introrse. Pistillate flowers with a two-celled, two-beaked ovary, the carpels produced into a long, recurved, persistent style. The ovaries all more or less cohere and harden in fruit. There are many ovules but few mature.[8]

The distinctive compound fruit is hard, dry, and globose, 1–1.5 inches in diameter, composed of numerous (40-60) capsules.[11] Each capsule, containing one to two small seeds, has a pair of terminal spikes (for a total of 80-120 spikes). When the fruit opens and the seeds are released, each capsule is associated with a small hole (40-60 of these) in the compound fruit.

The fruit is a multicapsular spherical head and hangs on the branches during the winter. The woody capsules are mostly filled with abortive seeds resembling sawdust.[8] The seeds are about one-quarter of an inch thick, winged, and wind-dispersed. Goldfinches, purple finches, squirrels, and chipmunks eat the seeds of the tree.[16] The seeds stratify within 30–90 days at 33°–41 °F or soaked in water for 15–20 days.[15] The long-stemmed fruit balls of Liquidambar resemble those of the American sycamore or buttonwood (Platanus occidentalis), but are spiny and remain intact after their seeds are dispersed; the softer fruits of Platanus disintegrate upon seed dispersal. The long-persisting fallen spiked fruits can be unpleasant to walk on; sweet gum is banned in some places for this reason.[12] In abundance, they can leave a lawn lumpy. The winter buds are yellow brown, one-fourth of an inch long, acute. The inner scales enlarge with the growing shoot, becoming half an inch long, green tipped with red.[8]

Sweetgum (Liquidambar styraciflua) is one of the most important commercial hardwoods in the Southeastern United States. Its wood is bright reddish brown (with the sapwood nearly white) and may have black grain in the heartwood; it is heavy, straight, satiny, and close-grained, but not strong. It takes a beautiful polish, but warps badly in drying. The wood has a specific gravity of 0.5910. It is too liable to decay for outdoor use.[8]
In the carpentry industry, the timber is referred to as satin walnut[10] and is one of the most important materials for plywood manufacturers. It is used for furniture, interior trim, railroad ties, cigar boxes, crates, flooring, barrels, woodenware, and wood pulp.[19] It is also used for veneer for plywood. The wood is very compact and fine-grained, the heartwood being reddish, and, when cut into planks, marked transversely with blackish belts. Sweetgum is used principally for lumber, veneer, plywood, slack cooperage, fuel, and pulpwood. The lumber is made into boxes and crates, furniture, cabinets for radios, televisions, and phonographs, interior trim, and millwork. The veneer and plywood, (typically backed with some other kind of wood which shrinks and warps less) are used for boxes, pallets, crates, baskets, and interior woodwork. It was formerly used in the interior finish of railroadsleeping cars. Being readily dyed black, it is sometimes substituted for ebony for such uses as inexpensive picture frames.[8][20]

The tree's gum resin, for which the tree is named, exudes from the bark of the tree when wounded.[16] It has many names, including liquid amber or copalm balsam. It is a kind of native balsam, or resin, resembling turpentine. It may be clear, reddish, or yellow, with a pleasant smell like ambergris. As the resin ages, it solidifies, the form in which it was historically exported in barrels. The resin is produced by stripping, boiling, and pressing the tree's bark.[12] A similar resin, known as styrax, is produced from another species of Liquidambar, the oriental sweet gum, L. orientalis, of the eastern Mediterranean, especially Turkey, the original source of the chemical styrene from which the resin was reputed to be an excellent balsam for mollifying and consolidating,[citation needed] and good against such conditions as sciatica and weakness of the nerves.[citation needed] The Aztecs believed styrax to have medicinal properties;[12] mixed with tobacco, the gum was once used for smoking at the court of the Mexican emperors.[21] Styrax is still used to make chewing gum and also as an ingredient for the tobacco and perfume industry, including the making of perfumes and cosmetics. The styrax gum is also used medicinally to treat coughs, wounds, and dysentery.[16] In fact, the old-fashion Friar's basalm contains liquid styrax and is used as an inhalant in bronchial upsets.[10]

Liquidambar styraciflua is a popular ornamental and forestal tree, cultivated for its distinctive foliage and intense autumn colors. It is commonly grown throughout its native North American range as well as many other temperate parts of the world. The species grows best in moist, acidic loam or claysoil, and tolerates poor drainage. It typically grows with other coastal plain species such as willow oak and sweetbay magnolia.[16] Its salt tolerance is moderate. Chlorosis can develop on alkaline soil, especially where organic matter is low. Also, the American sweetgum tree doesn't grow well in shady areas.[11]

The imperfect fungus Tubakia dryinaSutton is a leaf parasite reported to occur on a wide range of host plants, including species of Sweet Gum (Liquidambar styracifluaL.). Limber and Cash reported that leaf spots produced by this pathogen on several different genera of forest trees were 2–5 mm diameter with regular margins. During the summer of 1994 in the Nacogdoches County area of Texas, a prominent leaf spot on Sweet Gum was widespread. Infected leaves had numerous necrotic lesions, each surrounded by a reddish halo. The lesions tended to merge resulting in large areas of dead tissue. Infection and fungal development of T. dryina were investigated on leaves of sweet gum using a combination of microscopic techniques. T. dryina infection on Sweet Gum has been associated with the disease red leaf spot. Results of this investigation indicate that T. dryina can penetrate leaf tissue directly, thus having the ability to initiate infection on both upper and lower leaf surfaces. In other regions of the U.S., Sweet Gum populations may not be as susceptible to local populations of this fungus. Environmental stress factors may also be involved, as reports have indicated that herbicide application and chlorosis caused by iron deficiency may increase susceptibility of T. dryina. Tannins (a type of biomolecule found in trees to protect it from fire, insects, and bacteria) have been reported to occur in healthy tissue of a variety of plants including sweet gum. They may prevent pathogen invasion by inhibiting fungal enzyme activity. Although cells of healthy sweet gum tissue appear rich in tannins, these materials apparently were not effective in preventing fungal colonization by T. dryina.[26]